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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<title>The GHC Commentary - why we have <tt>ForeignPtr</tt></title>
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<h1>On why we have <tt>ForeignPtr</tt></h1>
<p>Unfortunately it isn't possible to add a finalizer to a normal
<tt>Ptr a</tt>. We already have a generic finalization mechanism:
see the Weak module in package lang. But the only reliable way to
use finalizers is to attach one to an atomic heap object - that
way the compiler's optimiser can't interfere with the lifetime of
the object.
<p>The <tt>Ptr</tt> type is really just a boxed address - it's
defined like
<pre>
data Ptr a = Ptr Addr#
</pre>
<p>where <tt>Addr#</tt> is an unboxed native address (just a 32-
or 64- bit word). Putting a finalizer on a <tt>Ptr</tt> is
dangerous, because the compiler's optimiser might remove the box
altogether.
<p><tt>ForeignPtr</tt> is defined like this
<pre>
data ForeignPtr a = ForeignPtr ForeignObj#
</pre>
<p>where <tt>ForeignObj#</tt> is a *boxed* address, it corresponds
to a real heap object. The heap object is primitive from the
point of view of the compiler - it can't be optimised away. So it
works to attach a finalizer to the <tt>ForeignObj#</tt> (but not
to the <tt>ForeignPtr</tt>!).
<p>There are several primitive objects to which we can attach
finalizers: <tt>MVar#</tt>, <tt>MutVar#</tt>, <tt>ByteArray#</tt>,
etc. We have special functions for some of these: eg.
<tt>MVar.addMVarFinalizer</tt>.
<p>So a nicer interface might be something like
<pre>
class Finalizable a where
addFinalizer :: a -> IO () -> IO ()
instance Finalizable (ForeignPtr a) where ...
instance Finalizable (MVar a) where ...
</pre>
<p>So you might ask why we don't just get rid of <tt>Ptr</tt> and
rename <tt>ForeignPtr</tt> to <tt>Ptr</tt>. The reason for that
is just efficiency, I think.
<p><small>
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Last modified: Wed Sep 26 09:49:37 BST 2001
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